Poster D60, Wednesday, August 21, 2019, 5:15 – 7:00 pm, Restaurant Hall
Motor imagery of speech increases activation of tongue motor area – a Transcranial Magnetic Stimulation study
Gwijde Maegherman1, Helen E. Nuttall2, Joseph T. Devlin1, Patti Adank1;1University College London, 2Lancaster University
Motor imagery is hypothesised to involve motor cortex activation in the absence of muscle movement. Most research in this area focuses on manual actions, with a particular emphasis on motor rehabilitation. However, people engage in other types of imagery, such as imagery of speech, yet this type of orofacial motor imagery has not been the focus of intensive research. Motor imagery of speech is similarly thought to involve activation of speech motor areas - those of the articulators. In the current study, we investigated the activation of tongue motor cortex by testing if motor-evoked potentials (MEPs) would be facilitated during motor imagery of speech. Finding such facilitation would have important implications for theories of auditory hallucination and elucidate the role of forward models of speech. Twenty participants took part in an experiment using transcranial magnetic stimulation (TMS) to evoke 240 MEPs from tongue motor cortex during three conditions: articulation observation, articulation imagery, and a baseline condition. The articulation observation condition involved listening to an exemplar of the non-native consonant cluster /tr/. Participants had been trained to produce this cluster prior to the start of the experiment, even though they were not required to actually produce it. The articulation imagery condition involved imagining producing this sound, while the baseline condition instructed participants to maintain their baseline position. Participants were shown a countdown, ending on a cue to perform the required action. Electrodes were fitted to a tongue depressor which the participants pushed lightly onto the roof of the mouth during the TMS session. MEPs were evoked at 200ms and 500ms post-cue to assess the temporal characteristics of motor activation during imagery. We performed a Repeated-Measures Analysis of Variance (ANOVA) on the MEP data to establish the effect of condition (articulation observation, articulation imagery, baseline) and time point (200ms & 500ms) on the size of the area-under-the-curve of the MEPs. The results showed increased MEPs in the articulation imagery condition versus the other conditions, but did not find such a difference for the articulation observation condition. No effect of time point was found and no interactions. This study aimed to examine whether we could see differences in tongue motor cortex activation in a task involving tongue articulation observation, articulation imagery and a baseline condition. We conducted an experiment to compare these three conditions using MEPs, which show facilitation of motor cortex of relevant effectors. The results show a primary effect of condition, indicating that articulation imagery facilitated MEPs more than in the articulation observation or baseline conditions. Our results suggest that motor imagery of speech involves motor plan activation in primary motor cortex. This has important implications for the understanding of generation of forward models in speech processing, suggesting that motor simulation may take place not only in somatosensory areas but also articulator-specific motor areas. Additionally, these results help clarify the origins not only of egocentric speech imagery but also the imagined perception of speech from a non-self source, as in auditory hallucinations.
Themes: Language Production, Speech Motor Control
Method: Neurostimulation